The present application relates to nuclear magnetic resonance (NMR) imaging and, more particularly, to a novel slotted-tube radio-frequency (RF) resonator having an elliptical cross-section, for use in obtaining NMR images in a NMR imaging system.
There has hitherto been wide speculation in NMR publications (such as "Radio Frequency Coil Technology In NMR Scanning", D. Hoult, Proc. Internat. Symp. on NMR Imaging (1982), pages 33-39; "NMR Imaging and Biomedicine", P. Mansfield et al., Academic Press (1982), pages 174-187; "Positron Tomography and Nuclear Magnetic Resonance Imaging", G. Brownel et al., Science, (1982) issue 215, pages 619-626; "NMR Imaging in Medicine", I. Pykett, Scientific American (1982) issue 246, pages 78-88; and other publications) that hydrogen head and/or body imaging would be unfeasible in a static NMR imaging field of magnitude much above about 0.5 Tesla (T), at least in part due to the problem of providing suitable NMR imaging coils at the Larmor frequencies, typically in the range from about 20 MHz. to about 100 MHz., associated with the greater static field magnitudes. It is well known that conventional "half turn" saddle coil, Helmholtz and solenoidal geometries are generally unsuited and precluded for head and body-sized NMR imaging coils at frequencies above about 30 MHz. due to their self-resonant properties, and thus would be unusable at the approximately 64 MHz. imaging frequency for .sup.1 H resonance in an imaging system utilizing a static field of about 1.5 T magnitude.
A slotted-tube resonator has been used at an even higher frequency of 300 MHz., for small-sample .sup.1 H NMR analysis, but this resonator (described by D. Alderman et al., in "An Efficient Decoupler Coil Design which Reduces Heating in Conductive Samples in Super Conducting Spectrometers", Volume 36, J. of Mag. Resonance, pp. 447-451, 1979), has such a small working volume diameter (e.g. about 0.72") as to preclude access of the human head, extremities and/or torso therein for imaging purposes.
It is also well known that the frequency dependence of the signal-to-noise ratio (S/N) in an NMR imaging system in which system noise contributions and dielectric losses in the sample have been minimized, is given by the formula: EQU (S/N).alpha.F.sup.2 /(aR.sub.c.sup.2 F.sup.1/2 +bF.sup.2 R.sub.s.sup.5).sup.1/2
where R.sub.c and R.sub.s are the respective coil and sample radii, a and b are constants and F is the Larmor (NMR) frequency of the sample in the particular magnetic field magnitude utilized. The F.sup.2 term in the numerator reflects the frequency dependence of the NMR imaging return signal, while the two terms in the denominator represent noise contributions respectively due to coil losses and inductive losses in the sample. The coil loss noise-inducing term (aR.sub.c .sup.2 F.sup.1/2) can be considered optimal when its noise contribution is insignificant compared to the inductive losses in the body sample being imaged. In an experiment utilizing a typical saddle coil for head imaging at about 5 MHz., the observed losses in the imaged object, e.g. the patient's head, were only about 0.4 times the coil losses. Thus, the coil noise contribution in this experiment is not insignificant compared to the body loss contribution and coil noise cannot be considered optimized.
Accordingly, it is desirable to provide a radio-frequency coil for NMR imaging which will not only be capable of operation at the Larmor frequencies (between about 20 MHz. to about 100 MHz.) associated with static magnetic fields of magnitude greater than about 0.5 Tesla, but will also have a relatively insignificant loss contribution compared to the losses introduced by the object being imaged. The ratio of the RF coil quality Q factor measured with the NMR coil substantially filled or loaded by the body (Q.sub.L) can be compared to the Q factor measured with the coil empty or unloaded (Q.sub.U) and is a measure of the relative noise contribution from the RF coil. Therefore, a high Q.sub.U Q.sub.L ratio is most desirable to optimize the signal-to-noise ratio for NMR imaging.